1. Field of the Invention
The present invention relates to digital to analog (D/A) converters, and more particularly to a high accuracy D/A converter using parallel P and N type resistor ladders.
2. Description of the Related Art
D/A converters using R-2R or segmented R (resistor/resistive) strings or ladders are relatively common. The resistor string or ladder is coupled to a common node (e.g., ground), a reference voltage, and an output node, and is typically segmented or the like forming multiple switch nodes. A series of switches, such as single-pole, double throw (SPDT) switches or the like, are each coupled to a corresponding switch node and responsive to a corresponding one of multiple data bits of an input digital value. Each “bit” switch couples its corresponding switch node between two different voltages or voltage nodes, such as between ground and the reference voltage, based on the corresponding data bit. The collective state of the data bits dictate the voltage level of the output node thereby representing the input digital value.
The accuracy of the D/A converter is directly affected by the matching of the resistors and also by the resistive matching of the two halves of each SPDT switch. Concerning resistor matching, diffused resistors have a strong voltage coefficient; the resistance of a diffused resistor increases as the voltage is increased between the resistor and the substrate “Well” in which it is contained. Thus, a simple diffused resistor was usually not considered sufficiently accurate to achieve an accurate D/A converter. High value poly or deposited metal resistors are available in some processes, but add significant extra cost. High value poly or deposited metal resistors are not available in many common processes. Concerning switch matching, the relative switch impedances between the two switching paths should match as closely as possible. A possible alternative solution is to make the switches have negligible resistance compared to the resistors of the ladder, such as by using very large switching devices. Such alternative configuration is undesirable due to layout area and potential parasitic capacitance.
The present disclosure addresses both resistance matching accuracy and switch matching accuracy.